I dont trust bungs and never used them. What I do is shut mains water off, open tap, start draining, Rewire new valve whilst its draining, You will hear when the water level drops below the 3 port so as soon as it drains enough swap it, shut off DOC start filling whilst I tidy up turn it on leave for 20 mins whilst drinking your tea then away you go. I very rarely get airlocks either doing it this way.